Messages - kazethefox

You could run with it- have each caster draw a hand of cards at the start of battle, and give them the ability to 'refocus' (discard all and draw up to their hand limit). This also lets you have different types of casters different hand sizes or more/less special mana in their decks. (I'd make deck size static though)

Null aspects could be gained by being injured/enemy spells/ etc., and you could even give the different mana types abilities taken from deck building games. Air aspect could let you peek at the top card of your stack and either discard or put it back, playing 2 fire aspects lets you draw a card immediately, or death aspects let you discard one of any card to draw another (good for getting rid of nulls).

Heck you could even make magic items change the decks too- a staff of the forest changes 2 of your mana into forest type, or a vest of mana protection that prevents one non-null type of mana from being put into your deck. It seems to be a very flexible way to run magic.